In recent years, the aquaculture industry has developed remarkably. However, frequent occurrence of viral or bacterial diseases in association with such development has caused great economic losses.
Antibiotics or synthetic antibacterial agents have been used as therapeutic drugs for such bacterial diseases, but satisfactory therapeutic effects have failed to be attained, due to emergence of bacteria resistance to such an antibacterial substance. Moreover, retention of such a therapeutic drug in fish bodies has caused public health problems. Therefore, keen demand has arisen for establishment of measures for fish diseases without use of an antibacterial substance.
In view of the foregoing, development of vaccines is an important issue. Regarding viral diseases, iridoviral disease in, for example, yellowtail, amberjack, and red sea bream has caused great economic losses. However, a commercially available vaccine for such a viral disease has reduced occurrence of the disease. Regarding bacterial diseases, for example, streptococcosis in yellowtail or bastard halibut, and edwardsiellosis in bastard halibut or sea bream, have caused great economic losses. Among these bacterial diseases, the occurrence of streptococcosis in yellowtail has been reduced, by virtue of development of a vaccine therefor.
However, vaccines for bacterial diseases in bastard halibut or sea bream have not yet been satisfactory. Hitherto, attempts have been made to develop a vaccine for edwardsiellosis in bastard halibut, which has caused great economic losses (Non-Patent Document 1). When formalin-killed cells of a bastard halibut-derived Edwardsiella tarda strain which is a pathogen are given as a vaccine to a bastard halibut, an antibody to the pathogen is produced in the blood of the bastard halibut, and leukocytes actively phagocytize E. tarda. However, the pathogen resists the bactericidal action of leukocytes and survives in leukocytes; i.e., the vaccine does not exhibit the effect of preventing the disease. Thus, a vaccine effective for edwardsiellosis in bastard halibut has not yet been developed so far. For reasons similar to those described above, a vaccine effective for edwardsiellosis in sea bream has not yet been developed.
“Streptococcosis” in bastard halibut collectively refers to diseases caused by Streptococcus iniae, S. parauberis, and Lactococcus garvieae. Although a vaccine for Streptococcus iniae infection has been developed, frequent occurrence of Streptococcus parauberis-induced disease has caused great economic losses. Therefore, demand has arisen for development of a vaccine for complete prevention of streptococcosis. Meanwhile, in aquaculture farms, edwardsiellosis and streptococcosis co-occur frequently in the form of mixed infection. Therefore, demand has arisen for development of a combination vaccine for exhibiting proper effects on these two diseases.    Non-Patent Document 1: Mekuchi, et al.; Fish Pathology, 30 (4), 251-256: 1995